RESEARCHES ON YEASTS. 91 



No. I yeast, is still far inferior to the latter as regards 

 stability. 



In all the breweries where these two yeasts were tried, 

 they always behaved in the manner described ; the chief 

 characters upon which stress has been laid were the same in 

 all cases. 



The term stability as here employed has reference only to 

 the formation of yeast sediment, and not to bacterial disease. 

 If we examine the matter more closely, we find that the 

 yeast sediment may consist partly of the culture species, 

 which alone has brought about the whole of the fermentation, 

 or was in preponderance from the commencement, and partly 

 of wild yeasts. We are not here considering cases in which 

 several culture yeasts were present together. A yeast which 

 is to yield a stable beer must be a species which multiplies 

 only to a small extent in the finished lager beer, and which 

 can keep competing organisms in check during the fermenta- 

 tion. With regard to the last point, the chief reason why 

 certain species excel in this respect must be sought in some 

 cases in the fact that they are better able to avail themselves 

 of the conditions of nutrition, and especially the oxygen, than 

 are the competing organisms, and, on the other hand, in 

 other cases, that during their multiplication they secrete 

 substances which act as poisons. 



In making comparative tests on stability in the sense 

 employed above, the beers must be bright, and should contain 

 only a small number of yeast cells ; strictly speaking, there 

 should be the same number of cells in the different samples. 

 There are low-fermentation yeasts which give a very stable 

 beer, but in which the brightening in the lager cellar is slow, 

 and inversely there are several species which yield instable 

 beers, but produce a rapid brightening in the lager cellar. 

 At a given time, when the latter beers are already bright, the 

 former will often be found to contain an infinite number of 

 yeast cells, and this circumstance is sufficient to render these 



